Mmmm! I can't help you much but I will put my two peneth in as they say...... Is it possible to change the door to the other end of the greenhouse, then you could have a small opening in the glass.....grapes need a bit of looking after, like not touching the grapes with your hands plus using scissers & tweezers to cut out some of the grapes on the bunch so they have more room to swell also I think you need to get rid of that Ivy, I sure its not helping?
Hopefully someone will come in and tell you more about your grapes as that is all I remember.
Good Luck Jackie :o))

Two things I notice on the photograph.

You need to thin each bunch of grapes, all of those very small grapes deep inside the bunch will never ripen, so they just rot and spoil the whole bunch. The Idea is to let air circulate through the bunch and because there are less grapes competing for water and you get bigger grapes.

I don't think the Ivy is helping.

Snap Ladyessex1

Ha! Ha! Ha! Yes we crossed Pimpernel :o))
40yrs ago in an old house I grew a black grape in my greenhouse and cared for it, one has to have Patience as well lol, the grapes where small & tough skinned + full of pip's lol but didn't have the heart to get rid of it, looked good in the greenhouse.

thankyou Ladyessex1 & Pimpernel. I'll be out with my nail scissors then to thin them out. Oooooooh but I can't sacrifice the ivy I'm afraid. Maybe I'll just cut it back.

Jenny if you want to have a productive grape vine the the ivy needs to go or be moved. The ivy is smothering the vine! You also need to prune the vine back each year... Rather than me trying to give advice google pruning a grape vine and you will get very clear instructions.

Thank you Moon growe - I've googled it but there are dozens of tips for new vines not any for old dears like mine. I will however look again thank you.

Jenny pruning your vine is exactly the same whether it is new or old. You need to cut back to a certain number of buds on each branch each year. I used to know all this by rote but have forgotten so you will need to get the info on. But, and it is a huge 'but', unless you get rid of that ivy you may as well give up - it is strangling the vine!

Jenny pruning your vine is exactly the same whether it is new or old. You need to cut back to a certain number of buds on each branch each year. I used to know all this by rote but have forgotten so you will need to get the info on. But, and it is a huge 'but', unless you get rid of that ivy you may as well give up - it is strangling the vine!

Thankyou again Moon Growe. The ivy does not entangle the vine in any way. It grows to the side of it and hangs all around it but it is not strangling it in the slightest. It has never ever attempted to cling to the vine - almost as if it understands that it shouldn't do it. Thank you for your comment.

Note also that the grapes in the grocers have often been sprayed with gibberellin to force the fruit to grow larger. Without the hormones, the fruit is naturally smaller.

Thankyou Tugbrethil I really did not know this.